In essential fatty acid (EFA)-deficinet rats and in rats with liver EFA depleted by refeeding a high-carbohydrate diet after fasting, large increases occur in hepatic levels of "lipogenic" enzymes, i.e., glucose-6-phosphate dehydrogenase, acetyl-CoA carboxylase, and fatty acid synthetase. Fatty acid synthesis in vivo is also elevated. These increases in enzymes levels (amount of enzyme protein) and in fatty acid synthesis are reversed by feeding a source of EFA for ca. 3 days, but not by feeding "non-essential" fatty acids, such as stearic acid, palmitic acid, or oleic acid. These "non-essential" fatty acids rapidly decrease de novo fatty acid synthesis in cultured cells when these fatty acids are added to fat-free culture media. Thi "short-term" inhibition (regulation) does not involve changes in enzyme concentrations. EFA could act in regulation of hepatic fatty acid synthesis (a) through their role as precursors of prostaglandins which influence actions of hormones that act to regulate lipogenesis; (b) through regulating synthesis of enzymes such as fatty acid synthetase (Flick et al., J. Biol. Chem. 252, 4242, 1977) or altering activities of hormone-dependent, membrane-associated enzymes through changes in membrane fatty acid composition; (c) through acting in short-term inhibition of fatty acid synthesis; (d) through affecting structure and metabolism of triglyceride-rich lipoproteins which may play an important role iin short-term regulation. Our approach is to investigate (c) and (d) by studying effects of C20-unsaturated fatty acids on fatty acid synthesis in isolated hepatocytes, by investigating triglyceride-rich lipoprotein secretion into and removal from blood in rats fed different fat and EFA intakes, and by comparing physical properties of phospholipids with a saturated fatty acid at the 1-position and C18- and C20-unsaturated fatty acids at the 2-position.